The present invention relates to an adhesive film for electronic parts, for example, an adhesive film for electronic parts in a tape BGA (Ball Grid Array) or a xcexc-BGA (trade name), for adhering a circuit board to an IC chip or to a reinforcing metal board, for adhering an IC chip to an IC chip, and for reducing stress caused by thermal expansion difference between a circuit board and an IC chip or a reinforcing metal board.
Recently, xcexc-BGA (produced by Tessera Inc.) and tape BGA packages have drawn attention due to the high speed property thereof, applicability to many pin forms, or the like, as packages for semiconductor memory or packages for ASIC, etc. The xcexc-BGA is produced by bonding an IC chip to a flexible tape substrate via an adhesive layer and by attaching soldering balls thereto. The tape BGA is produced by bonding a reinforcing metal board to a flexible tape substrate via an adhesive layer and by attaching soldering balls thereto. These packages must act to reduce stress caused by thermal expansion difference between a circuit board and an IC chip or a reinforcing metal board.
As a convenient adhesive agent for reducing stress in the xcexc-BGA, silicone type resin is employed. For example, liquid silicone resin is printed on a circuit board, an IC chip is adhered on a surface thereof, and the silicone resin is cured by heating. Thereafter, this laminate is sealed by resin, thereby completing an IC package. However, in the case in which the silicone resin is printed, since it is difficult to obtain uniformity of printing amount (thickness, etc.), productivity is low, or the like, and an adhesive agent shaped as a film is preferably employed. The adhesive film must be in a semi-cured state (hereinafter, this stage of the thermosetting resin is referred to as the xe2x80x9cB-stagexe2x80x9d) so as to facilitate handling. In order to adhere by an adhesive film in the B-stage, this film is punched out in a specific shape by a die, is mounted on a tape substrate, is pressed thereon on an IC chip, and is cured by heating. Thereafter, this laminate is sealed by resin, thereby completing an IC package. As an adhesive film, in addition to silicone resin film, a B-stage adhesive film consisting of butyl acrylate/phenol resin/epoxy resin as a base component, and a B-stage adhesive film consisting of butyl acrylate/phenol resin/epoxy resin as a base component, are provided on surfaces of a polyimide film, such as Upilex film, produced by Ube Industries, Ltd., or Kapton E type produced by Du Pont-Toray Co., Ltd., or the like.
For example, the silicone resin is characterized in that its melting point is in the vicinity of xe2x88x9250 xc2x0 C. and its elastic modulus change depending on temperature change is small. Therefore, it is said that the xcexc-BGA employing the silicone resin causes few problems such as imperfect joining, short circuiting, etc. in ball junctions or copper leads in thermal cycle tests that test the reliability of IC chips. However, since conventional silicone resin film is produced by interrupting a reaction in the step of curing liquid silicone by heating, it is difficult to produce a constant semi-cured state thereof. Copper patterns formed on a circuit board must be embedded so that unevenness thereof is filled with resin. In the case in which embedding in resin is not adequate, moisture, etc., may seep into a space between a surface and the resin and be rapidly expanded by heating (solder reflow) when a package is assembled on a motherboard, whereby a steam explosion (popcorn phenomenon) may occur, resulting in destruction of the package, loss of flatness, distortion, or the like, and further contact of solder balls or imperfect contact with the motherboard, etc., may occur. However, it is difficult to embed a copper pattern since silicone resin film which is cross-linked to some degree is approximately an elastic body. As a result, it is necessary to heat it to a high temperature of 200xc2x0 C. or more. In the case in which cross-linking thereof is restricted so as to improved embedding, since silicone resin has a low molecular weight and is fluid, it may be difficult to control the thickness of the silicone film, the adhesive area on a tape, etc. In addition, since restricted cross-linking silicone resin is very soft, accuracy of punching by a die and adhesion of resin to the die may be problems. Furthermore, since a silicone resin film is continuously processed while feeding, it is necessary to restrict adhesion at ordinary temperatures, whereby it may be difficult to adjust the processing during feeding in the semi-cured state.
With respect to a B-stage thermosetting adhesive film consisting of butyl acrylate/phenol resin/epoxy resin as a base component, the thickness of an adhesive layer is required to be about 150 to 200 xcexcm so as to allow copper leads to be bent into a S-shape. The B-stage adhesive film consisting of butyl acrylate/phenol resin/epoxy resin as a base component is soft at ordinary temperatures, so there is a problem in that punching quality by a die may be inferior. In an adhesive film provided with B-stage adhesive films consisting of butyl acrylate/phenol resin/epoxy resin as a base component on both surfaces of a polyimide film, imperfect contact at solder ball joint portions, short circuiting at copper leads, etc., readily occur in reliability tests of IC packages, for example, in thermal cycle tests at about xe2x88x9230 to 120xc2x0 C.
Furthermore, with respect to the tape BGA, there is a problem in that electrical resistance rises at solder ball joint portions in thermal cycle tests after assembly on a substrate. This stress at solder ball joint portions in thermal cycle tests is also caused by thermal expansion difference among a reinforcing metal board, an organic film of a tape substrate, and a circuit board.
Therefore, it is an object of the present invention to provide an adhesive film for electronic parts, for example, an adhesive agent in a xcexc-BGA (trade name) or a tape BGA package, in which the embedding and the adhesion to a copper pattern on a circuit board are superior, in which feeding and punching quality in the film state are superior, and which can reduce stress caused by thermal expansion difference between a circuit board and a reinforcing metal board or an IC chip.
According to the results that the inventor has obtained from various research with regard to adhesive films for electronic parts, in which punching quality, feeding, and embedding are superior, and in which stress reduction is sufficient, the above properties are divided into each layer by providing adhesive layers on both surfaces of a resin layer, thereby attaining the present invention. Since the results quantitatively analyzed properties of an adhesive layer by which embedding a copper pattern is superior and feeding in a production device is smooth, the inventor has found that the above properties are satisfied when the dynamic modulus of elasticity at xe2x88x9230 to 125xc2x0 C. of a resin layer provided in an adhesive film ranges from 1 to 30 MPa. Therefore, the adhesive film for electronic parts according to the present invention is characterized by comprising a resin layer in which the dynamic modulus of elasticity at xe2x88x9230 to 125xc2x0 C. ranges from 1 to 30 MPa.
In addition, the adhesive film for electronic parts according to the present invention is preferably that an adhesive agent for forming the adhesive layer has a melting temperature ranging from 50 to 200xc2x0 C. In the case in which the melting temperature of the above adhesive agent is 200xc2x0 C. or more, an adhesive layer must be adhered to a circuit board, an IC chip, or a metal board by heating to a high temperature since inadequate embedding is readily caused, whereby the following problems may be arise.
{circle around (1)} A circuit board is distorted by heating, and a protective film provided on a surface of an adhesive layer shrinks by heating, whereby it is difficult to control the adhesive area.
{circle around (2)} An adhesive agent employed in a circuit board is spread out by thermal expansion, whereby leads are contaminated and adhesion of an adhesive layer on a circuit board are decreased.
{circle around (3)} Moisture evaporated from a circuit board enters into an adhesive layer, whereby pores (voids) are produced in the adhesive layer.
{circle around (4)} Since thermal expansion difference between a circuit board and an IC chip is large when the IC chip is adhered to an adhesive layer by heating, in the case in which a package is produced by sealing resin in this state, distortion and internal stress therein remain in the package.
{circle around (5)} A partial curing reaction occurs (for example, at a surface portion) when an adhesive layer is heated, whereby adhesion to an IC chip is decreased.
In the case in which the melting temperature of the above adhesive agent is 50xc2x0 C. or less, adhesion appears in an adhesive layer at ordinary temperatures and resin flow is readily caused, whereby punching quality and feeding is impaired. In the case in which the adhesive layer consists of a thermosetting adhesive agent, when the layer is rapidly adhered in an uncured state by heating, moisture readily evaporates from a circuit board and enters into the uncured adhesive layer, whereby voids easily form. Since this adhering by heating must therefore be performed over a long period so that moisture from a circuit board is evaporated after curing the adhesive layer, it is not practical.